Cornell demos single-atom transistor

ITHACA, N.Y.  Using a single cobalt atom as a switch, a research team at Cornell University has demonstrated a working transistor only 1.3 nanometers in length. Silicon transistors today are generally more than 100 nm long.

The researchers wrapped an organic alkyl chain around a single cobalt atom, lodged it in a minute fracture in a gold wire, then observed a series of step voltage increases indicating transistor behavior.

"We have a long way to go before we incorporate this transistor into electronic applications," said Paul McEuen, professor of physics at Cornell, "but we have demonstrated the potential for shrinking transistors well beyond the limits of conventional lithography." McEuen co-invented the technique with a former postdoctoral colleague, Hongkun Park, who is now a professor at Harvard University. Also contributing to the work were Cornell professors James Sethna, Dan Ralph and Hector Abruna.

Using individual atoms as transistors represents the ultimate in physical compactness for traditional logic states  "0" and "1." With current technology, bits are represented by above-100-nm-sized silicon transistors. Cobalt atoms were chosen for transistorization because they exist in two electrically distinguishable states that can be switched externally.

Additional chemistry then creates an octahedral wrapping (the carbon-hydrogen alkyl chain) and two long chain arms attached to opposite sides made from pyridine, a relative of benzene, that serve as I/O connections. The pyridine chains are terminated with a sulfur-hydrogen atom pair that forms a strong bond with the gold electrode.

The researchers used chemical self-assembly to create transistor specimens on a silicon substrate. The chip was created by growing a 30-nm layer of silicon dioxide on top of a doped silicon substrate, which plays the role of a back gate that switches all of the transistors simultaneously. Gold wires less than 200 nm wide, 400 nm long and 15 nm thick were fabricated directly on the chip with electron-beam lithography. The wires were then cleaned with acetone, methylene chloride and oxygen plasma. Immersing the chip overnight in a solution containing the chemical constituents of the cobalt transistors allows a thin monolayer of the devices to form on the gold electrodes.

Voltage jumps the crack

Cooling the chips to cryogenic temperatures, the team then applied a voltage to the wires that barely exceeded their load capacity. That operation generated hairline, 1.2-nm cracks by electromigration that the cobalt molecules could bridge with their extended long-chain bonding wires. The voltage point where cracks begin to appear can be detected by a sudden voltage drop to electron-tunneling levels. Because the gold wires are uniformly covered in chemical replicas of the atomic-scale transistors, the cracks are highly likely to be bridged by a transistor.

The electrical characteristics of the gap-bridging cobalt atom was then measured by plotting current vs. bias voltage while varying the back-gate voltage  just as a normal transistor would. Like normal transistors, the shorter ones worked better. Test results showed that the single-atom devices were switching single electrons, creating the ultimate in efficiency within a conventional electronic system.

McEuen said the researchers had not reproduced all of the characteristics of a transistor  amplification, for instance  and added that major technological hurdles are yet to be overcome in electronic applications. He suggested that highly sensitive sensors might be a more-near-term application, because environmental changes could switch the cobalt atom.

Shape of the future

Next, McEuen said, his group planned to try a new molecule that can switch between "1" and "0" by changing shape, rather than by losing or gaining an electron. This physical mechanism, used in place of an electronic switch, could have advantages in economy and efficiency. It's also never been done before, McEuen said  a good enough reason for basic research.

The cobalt atom device represents a new research direction for McEuen and Park. The two collaborated on carbon nanotube-based devices at the University of California, Berkeley, and then continued that work at Cornell and Harvard, respectively. Carbon nanotubes have been extensively studied as possible replacements for today's transistors since they have versatile electronic properties and can form ultrasmall gates that do not suffer from the insulation breakdown problems starting to plague the silicon MOSFET.

While at Berkeley, McEuen and Park hit on the idea of using fissures in gold wires as contact gaps and C60 "buckyball" molecules as a switching device. They published results on a buckyball single-electron transistor in the journal Nature in 2000.